Amyotrophic lateral sclerosis (ALS) is a progressive, neurodegenerative condition involving the loss of large motor neurons in the brain and spinal cord. It is characterized by progressive weakness, atrophy and spasticity, leading to paralysis and respiratory failure within five years of onset. Familial ALS accounts for 10% of all ALS cases; approximately 25% of these cases are due to mutations in the Cu/Zn superoxide dismutase gene (SOD1) [1]. To date 109 different mutations have been identified in the SOD1 gene; these span all five exons [2]. Besides very rare mutations in genes for heavy neurofilament chain (NFH), dynactin, vesicular binding protein 1 gene and the ALSIN gene, SOD1 is the only major ALS susceptibility locus identified. SOD1 is a mainly cytoplasmic enzyme that catalyzes the breakdown of superoxide ions to oxygen and hydrogen peroxide, which in turn is degraded by glutathione peroxidase or catalase to form water. Several lines of evidence argue that the mutant SOD1 protein is neurotoxic through an acquired, adverse function that entails both oxidative pathology and protein aggregation, with secondary disturbances of glutamate metabolism, mitochondrial function, axonal transport and calcium homeostasis [3]. That mutant SOD1 is toxic is strongly supported by the observation that transgenic expression of high levels of mutant SOD1 protein in mice produces a motor neuron disease phenotype, with age of onset and disease duration dependent on copy number [4].
To date, few therapeutic interventions have altered the motor neuron phenotype in the transgenic ALS mice. Although more than 100 small molecules have been tested to date, few have had even a marginal benefit (e.g. riluzole [5], celecoxib [6], arimoclomol [7]). By contrast, some forms of protein therapy have been beneficial. Thus, improvement in survival was produced by administering insulin-like growth factor 1 either transgenically [8] or through AAV2-delivery via IM injection and subsequent retrograde axonal transport to motor nerves [9]. Two other proteins that have shown therapeutic promise as neuroprotective agents are erythropoietin [10] and vascular endothelial factor (VEGF) [11, 12]. The latter is of interest because genetic analysis has implicated hypomorphic variants in the VEGF gene as a risk factor for ALS [13]. Moreover, mice that lack hypoxia-responsive promoter elements develop a slowly progressive motor neuron disease [14]. Subsequently, it was documented that lentiviral delivery of VEGF to the spinal cord of ALS mice delays death [15]. Two independent investigators have reported that infusion of VEGF into the cerebrospinal fluid in ALS mice [16] and rats [17] also slow the disease course.
Gene therapy is an emerging treatment modality for disorders affecting the central nervous system (CNS). CNS gene therapy has been facilitated by the development of viral vectors capable of effectively infecting post-mitotic neurons. The central nervous system is made up of the spinal cord and the brain. The spinal cord conducts sensory information from the peripheral nervous system to the brain and conducts motor information from the brain to various effectors. For a review of viral vectors for gene delivery to the central nervous system, see Davidson et al. (2003) Nature Rev. 4:353-364.
Adeno-associated virus (AAV) vectors are considered useful for CNS gene therapy because they have a favorable toxicity and immunogenicity profile, are able to transduce neuronal cells, and are able to mediate long-term expression in the CNS (Kaplitt et al. (1994) Nat. Genet. 8:148-154; Bartlett et al. (1998) Hum. Gene Ther. 9:1181-1186; and Passini et al. (2002) J. Neurosci. 22:6437-6446).
One useful property of AAV vectors lies in the ability of some AAV vectors to undergo retrograde and/or anterograde transport in neuronal cells. Neurons in one brain region are interconnected by axons to distal brain regions thereby providing a transport system for vector delivery. For example, an AAV vector may be administered at or near the axon terminals of neurons. The neurons internalize the AAV vector and transport it in a retrograde manner along the axon to the cell body. Similar properties of adenovirus, HSV, and pseudo-rabies virus have been shown to deliver genes to distal structures within the brain (Soudas et al. (2001) FASEB J. 15:2283-2285; Breakefield et al. (1991) New Biol. 3:203-218; and deFalco et al. (2001) Science, 291:2608-2613).
Several groups have reported that the transduction of the brain by AAV serotype 2 (AAV2) is limited to the intracranial injection site (Kaplitt et al. (1994) Nat. Genet. 8:148-154; Passini et al. (2002) J. Neurosci. 22:6437-6446; and Chamberlin et al. (1998) Brain Res. 793:169-175). Recent reports suggest that retrograde axonal transport of neurotrophic viral vectors, including AAV and lentiviral vectors, can also occur in select circuits of the normal rat brain (Kaspar et al. (2002) Mol. Ther. 5:50-56; Kasper et al. (2003) Science 301:839-842 and Azzouz et al. (2004) Nature 429:413-417. Roaul et al. (2005) Nat. Med. 11(4):423-428 and Ralph et al. (2005) Nat. Med. 11(4):429-433 report that intramuscular injection of lentivirus expressing silencing human Cu/Zn superoxide dismutase (SOD 1) interfering RNA retarded disease onset of amyotrophic lateral sclerosis (ALS) in a therapeutically relevant rodent model of ALS.
Cells transduced by AAV vectors may express a therapeutic transgene product, such as an enzyme or a neurotrophic factor, to mediate beneficial effects intracellularly. These cells may also secrete the therapeutic transgene product, which may be subsequently taken up by distal cells where it may mediate its beneficial effects. This process has been described as cross-correction (Neufeld et al. (1970) Science 169:141-146).
There is a need in the art for compositions and methods to treat dysfunction of the spinal cord that result in loss of motor function in human patients.